FIG. 1 shows a schematic view of a conventional liquid crystal display device using an edge light surface light source apparatus. A liquid crystal display device 11 is configured by a surface light source apparatus 12 and a liquid crystal panel 15.
The surface light source apparatus 12 has a point light source 18 using an LED arranged facing an end face (light incident surface) of a light guide plate 17 molded by a transparent resin, a diffusion plate 13 and two prism sheets 14 stacked on an upper surface (light outputting surface) of the light guide plate 17, and a reflection plate 16 arranged facing a lower surface of the light guide plate 17. The point light source 18 is mounted on a substrate 20. The liquid crystal panel 15 is arranged on the prism sheet 14 through a rim sheet 19 (black frame).
Light emitted from the point light source 18 enters into the light guide plate 17 from the end face of the light guide plate 17, propagates and expands in the light guide plate 17, and exits from substantially the entire upper surface of the light guide plate 17. The light outputted from the upper surface of the light guide plate 17 is transmitted through the diffusion plate 13 and the prism sheet 14 to illuminate the liquid crystal panel 15 from a back surface side. The light leaked from the lower surface of the light guide plate 17 is reflected by the reflection plate 16 to again return into the light guide plate 17, so that the light is reused.
Such a surface light source apparatus 12 is desired to have an even luminance, high luminance, inexpensive cost, large light outputting area (small area other than light outputting surface), and small thickness. In particular, in a case of being incorporated to a portable device, a demand for a thinner area light source apparatus 12 is increasing more and more with thinning of the portable device.
The size of each part of a general surface light source apparatus is as follows.
Sum of thicknesses of substrate and point light source 600 μm
Height of light outputting window of point light source 300 μm
Thickness of prism sheet 62 μm (per one sheet)
Thickness of diffusion plate 55 μm
Thickness of light guide plate 300 to 650 μm
Thickness of reflection plate 60 μm
Thickness of rim sheet 55 μm
The thickness of the surface light source apparatus is about 600 μm on the point light source side, and is between 539 μm and 889 μm on the light guide plate side even when the thickness of the rim sheet is excluded. Therefore, the thickness on the light guide plate side occupying the majority of the area of the surface light source apparatus is desirably reduced.
The light guide plate occupies the majority of the thickness of the surface light source apparatus (hereinafter, when referring simply to thickness of surface light source apparatus, this refers to the thickness on the light guide plate side of the surface light source apparatus). However, if the thickness of the light guide plate is made smaller than the height of the light outputting window of the point light source, the light that does not enter the light guide plate of the light emitted from the point light source increases and the light usage efficiency of the surface light source apparatus lowers. Thus, the thickness of the light guide plate is subjected to restriction by the height of the light outputting window of the point light source, and it is difficult to have the thickness of the surface light source apparatus smaller than the height of the light outputting window of the point light source. Similarly, if the light source is a cold cathode tube, it is difficult to have the thickness of the light guide plate smaller than the diameter of the cold cathode tube.
(Regarding Patent Document 1)
FIG. 2 is a side view of a liquid crystal display device 21 disclosed in Japanese Unexamined Patent Publication No. 5-53111 (Patent Document 1). In a surface light source apparatus 22 used in the liquid crystal display device 21, a tapered portion 25 is provided at the portion of small thickness of the light guide plate, that is, the end of the light guide plate main body 24 to efficiently enter light from a fluorescent tube 23 to the light guide plate having a thickness smaller than the fluorescent tube 23. The end face of the tapered portion 25 has a height substantially equal to the diameter of the fluorescent tube 23, and the fluorescent tube 23 faces the relevant end face. The light entered from the end face of the tapered portion 25 is introduced into the light guide plate main body 24 by being totally reflected at the front and back surfaces of the tapered portion 25, and outputted from the upper surface of the light guide plate main body 24 toward a liquid crystal panel 26.
The surface light source apparatus 22 disclosed in Patent Document 1 aims to introduce the light of the fluorescent tube 23 to the light guide plate without leakage. To this end, the height of the end face of the tapered portion 25 is made substantially equal to the diameter of the fluorescent tube 23, and the light of the fluorescent tube 23 is introduced to the tapered portion 25 without leakage. However, in the surface light source apparatus 22, the leakage of light at the tapered portion 25 cannot be prevented. Thus, the light leaked from the tapered portion 25 appears shining from an observer side, and hence the edge of the display unit (screen) of the liquid crystal display device 21 emits light at high luminance, and the quality of the display unit degrades.
A reason that the leakage of light from the tapered portion 25 cannot be prevented in the structure of the surface light source apparatus 22, for example, will be described with reference to FIG. 3. Assume that the light tends to leak out the most at the tapered portion 25. If the leakage of such light that tends to leak out the most is prevented, the leakage of light at the tapered portion 25 can be eliminated in the surface light source apparatus 22. The light that tends to leak out the most is a light ray L having a largest incident angle α of the light emitted from the fluorescent tube 23 and entered the tapered portion 25, and thus a structure in which the light ray L with the maximum incident angle α measured from a direction perpendicular to the end face of the tapered portion 25 does not leak out at the tapered portion 25 and the thickness of the light guide plat main body 24 is reduced as much as possible is considered. To obtain such a structure, a condition for totally reflecting the light ray L having the maximum incident angle α at the upper end (point A) of an inclined surface of the tapered portion 25, again totally reflecting at point B at the lower surface of the light guide plate, and reflecting at the upper surface (point C) adjacent to the tapered portion 25 of the light guide plate main body 24 is to be considered. In FIG. 3, a flat plate-shaped short portion is shown at the end face portion of the tapered portion 25, but this is shown for the sake of illustration, and the length can be assumed as infinitely short.
First, the maximum incident angle α of the light ray that entered the light guide plate is determined by,sin α=1/n  (equation 1)
(where n is index of refraction of light guide plate).
The incident angle at which the light ray L having the maximum incident angle α enters point A having an inclination angle θ is 90°−θ−α, and thus a condition that the light ray totally reflects at the inclined surface is,θ≦90°−2α  (equation 2).
The incident angle at which the light totally reflected at point A enters the lower surface of the tapered portion 25 is 90°−2θ−α, and thus a condition that the light ray totally reflects at point B of the lower surface is,θ≦45°−α  (equation 3).If the equation 3 is satisfied, the light totally reflected at point B will also be totally reflected at point C of the light guide plate main body 24.
Therefore, according to the equations 2 and 3, θ≦45°−α is to be satisfied in order for the light ray to be totally reflected at point A, point B, and point C.θ≦45°−α  (equation 4).
However, if the inclination angle θ of the tapered portion 25 is small, the light totally reflected at the lower surface of the light guide plate after being totally reflected at the upper end of the inclined surface of the tapered portion 25 may again enter the inclined surface of the tapered portion 25 and leak out from the tapered portion 25, and furthermore, the length of the tapered portion 25 becomes long if the inclination angle θ is small, and hence the inclination angle θ is desirably large as possible within a range satisfying the equation 4. Therefore, the inclination angle θ is a value large as possible at a limit satisfying the equation 4. In other words,θ=45°−α  (equation 5).
Assuming a height of the end face of the tapered portion 25 is T, a length of the tapered portion 25 is X, and a high and low difference of the inclined surface of the tapered portion 25 is Y, the length X and the high and low difference Y of the tapered portion 25 are as follows from FIG. 3.
                              X          =                    ⁢                                    T              ⁢                                                          ⁢                              cot                ⁡                                  (                                      α                    +                                          2                      ⁢                                                                                          ⁢                      θ                                                        )                                                      +                                          (                                  T                  -                  Y                                )                            ⁢                              cot                ⁡                                  (                                      α                    +                                          2                      ⁢                      θ                                                        )                                                                                                  =                    ⁢                                    (                                                2                  ⁢                  T                                -                Y                            )                        ⁢                          cot              ⁡                              (                                  α                  +                                      2                    ⁢                                                                                  ⁢                    θ                                                  )                                                              Y    =          X      ⁢                          ⁢      tan      ⁢                          ⁢      θ      Solving such equations for X and Y, and using the equation 5, the following equation 6 and equation 7 are obtained.
                    X        =                              2            ⁢                          a              ⁡                              (                                  1                  +                  a                                )                                      ×            T                                1            +                          2              ⁢              a                        -                          a              2                                                          (                  equation          ⁢                                          ⁢          6                )                                Y        =                              2            ⁢                          a              ⁡                              (                                  1                  -                  a                                )                                      ×            T                                1            +                          2              ⁢              a                        -                          a              2                                                          (                  equation          ⁢                                          ⁢          7                )            
where, a=tan α=tan (45°−θ).
The thickness t of the light guide plate mainbody 24 is expressed with the following equation 8.
                    t        =                              T            -            Y                    =                                                    (                                  1                  +                                      a                    2                                                  )                            ×              T                                      1              +                              2                ⁢                a                            -                              a                2                                                                        (                  equation          ⁢                                          ⁢          8                )            
Considering acrylic resin or polycarbonate resin (PC resin), which are representative light guide plate materials, for the light guide plate material, and calculating with the index of refraction of the light guide plate as,
n=1.49 (in the case of acrylic resin)
n=1.59 (in the case of polycarbonate resin),
the maximum incident angle α is, from the equation 1,
α=42.16° (in the case of acrylic resin)
α=38.97° (in the case of polycarbonate resin).
The inclination angle α of the tapered portion 25 is, from the equation 3,
θ=2.84° (in the case of acrylic resin)
θ=6.03° (in the case of polycarbonate resin).
In Patent Document 1, the height of the end face of the tapered portion 25 is described as T=4.10 mm, and thus the length X and the high and low difference Y of the tapered portion 25, and the thickness t of the light guide plate main body 24 can be obtained as below from the equations 6 to 8 using the value of the height T and the value of α. If the light guide plate material is acrylic resin, T=4.10 mm and α=42.16° (a=tan α=0.91), and thus
X=7.10 mm
Y=0.35 mm
t=3.75 mm,
Similarly, if the light guide plate material is polycarbonate resin, T=4.10 mm and α=38.97° (a=tan α=0.81), and thus
X=6.11 mm
Y=0.65 mm
t=3.45 mm.
FIG. 4 summarizes the above calculation results.
According to FIG. 4, the thickness t of the light guide plate main body 24 is 3.75 mm (in the case of acrylic resin) or 3.45 mm (in the case of polycarbonate resin). In the liquid crystal display device 21 disclosed in Patent Document 1, on the contrary, the thickness of the light guide plate main body 24 is described as t=2.2 mm with respect to the height of the end face of the tapered portion 25 of T=4.10 mm. The value t=2.2 mm is significantly smaller than the value (in FIG. 4) of the thickness t obtained through the above calculation, and thus the light will always leak out from the tapered portion 25.
Therefore, in the surface light source apparatus 22 disclosed in Patent Document 1, the leakage of light from the tapered portion 25 cannot be prevented. Alternatively, in the surface light source apparatus 22 disclosed in Patent Document 1, the leakage of light at least from the tapered portion 25 is not taken into consideration at all.
(Regarding Patent Document 2)
FIG. 5 is a perspective view showing a surface light source apparatus disclosed in FIG. 1 of Japanese Unexamined Patent Publication No. 2004-69751 (Patent Document 2). The surface light source apparatus 31 has a conical light introducing section 33 arranged at the end of a light guiding sheet 32, and a point light source 35 arranged facing the end face (light receiving section 34) of the light guiding section 33. In the surface light source apparatus 31 as well, the point light source 35 and the light receiving section 34 of the light introducing section 33 have a height of the same extent, so that the light of the point light source 35 is entered from the light guiding section 33 and introduced to the light guiding sheet 32.
The surface light source apparatus 31 disclosed in Patent Document 2 aims to introduce the light of the point light source 35 to the light guiding sheet 32 without leakage. Thus, the height of the light receiving section 34 is made substantially equal to the height of the point light source 35, and the light of the point light source 35 is introduced to the light guiding sheet 32 by the tapering of the light introducing section 33. However, in this surface light source apparatus 31 as well, the leakage of light at the conical light introducing section 33 cannot be prevented. Thus, the light leaked from the light introducing section 33 appears shining from the observer side, and hence the edge of the display unit (screen) of the liquid crystal display device emits light at high luminance, and the quality of the display unit degrades.
FIG. 6 is a view showing the cross-section of the light introducing section 33 and the light receiving section 34 taken along a perpendicular plane passing through an axis center of the light introducing section 33. The reason the leakage of the light from the light introducing section 33 cannot be prevented in the surface light source apparatus 31 of Patent Document 2 (FIG. 1) will now be described with reference to FIG. 6. Since the acrylic resin is used for the light guide plate material in the surface light source apparatus 31 of the Patent Document 2, the maximum incident angle α of the light ray L entered to the light guiding section 33 is α=42.16° from FIG. 4, and thus the inclination angle θ of the surface of the light introducing section 33 in this case is θ=2.84°. However, the height of the light receiving section 34 is 3 mm and the thickness of the light guiding sheet 32 is 1 mm in the surface light source apparatus 31 of Patent Document 2, and thus the length of the light introducing section 33 is X=20.16 mm if the inclination angle of the light introducing section 33 is θ=2.84°. Therefore, as shown in FIG. 6, the light totally reflected at the upper end of one inclined surface of the light introducing section 33 enters the other inclined surface at an incident angle of90°−(α+3θ)=39.32°.This incident angle 39.82° is an angle smaller than the critical angle (42.16°) of total reflection, and thus the light ray L that entered the other inclined surface leaks to the outside, as shown in FIG. 6.
Therefore, in the surface light source apparatus 31 disclosed in Patent Document 2, light of a certain extent can be confined but light of about a few dozen percent leaks out from the light introducing section 33, whereby the leaked light shines at the display surface and the quality of the liquid crystal display device degrades. In Patent Document 2, consideration is not made in preventing such leakage of light.
FIG. 3 of Patent Document 2 discloses a light guide plate in which the end face (light receiving section 34) of the light introducing section 33 is circular and the light introducing section 33 flatly expands and becomes thinner toward the side opposite to the light receiving section 34. This is shown in FIG. 7. In such a surface light source apparatus as well, the surface on the upper surface side of the light introducing section 33 and the surface on the lower surface side face each other substantially in parallel in view of the cross-section taken along K-K of FIG. 8, as apparent when seen from the light receiving section 34 side as in FIG. 8. Therefore, the light leaks out from the light introducing section 33 same as in the case of FIG. 1 of Patent Document 2 even in the light guide plate of such a mode.
(Regarding Patent Document 3)
FIG. 9 is a cross-sectional view of a surface light source apparatus disclosed in Japanese Unexamined Patent Publication No. 2005-285389 (Patent Document 3). A surface light source apparatus 41 has a light entering section 44 of step-form that gradually becomes thinner from the light incident surface side arranged at the end of the light guide plate main body 43 to efficiently enter the light to the light guide plate main body 43 having a thickness smaller than the light source 42. A light reflection plate 45 is arranged at the surface of the light entering section 44.
In the surface light source apparatus 41 of such a structure, the light ray leaked from the light entering section 44 is reflected by the light reflection plate 45 to re-enter the light entering section 44, and introduced to the light guide plate main body 43 by repeating reflecting at the interface of the light entering section 44 and the light reflection plate 45, as with a light ray L1 shown in FIG. 10.
However, in such a structure, some of the light is absorbed by the light reflection plate 45 and the usage efficiency of the light lowers as with a light ray L2 shown in FIG. 10. Furthermore, when fixing the light reflection plate 45 to the light entering section 44 with an adhesive, a considerable amount of light is absorbed and lost by the adhesive. Furthermore, since the light entering section 44 has a step-form, the reflected light ray leaks out from the light incident surface as with the light ray L1 of FIG. 10. Therefore, the usage efficiency of the light is not satisfactory in such structure compared to the surface light source apparatus that confines the light by total reflection.
In such a surface light source apparatus, the thickness of the light entering section 44 may increase by the thickness of the light reflection plate 45 since the light reflection plate 45 is arranged on the light entering section 44. Furthermore, the assembly cost in attaching the light reflection plate 45 to each surface of the light entering section 44 is also required, which leads to an increase in cost of the surface light source apparatus.    Patent Document 1: Japanese Unexamined Patent Publication No. 5-53111    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-69751    Patent Document 3: Japanese Unexamined Patent Publication No. 2005-285389